In industrial settings, control systems are used to monitor and control inventories of industrial and chemical processes, and the like. Typically, the control system performs these functions using field devices distributed at key locations in the industrial process that are coupled to control circuitry in the control room by a process control loop.
Field devices are used by the process control and measurement industry for a variety of purposes. Usually, such devices have a field-hardened enclosure so that they can be installed outdoors in relatively rugged environments and be able to withstand climatological extremes of temperature, humidity, vibration and mechanical shock. Field devices also can typically operate on relatively low power. For example, field devices are currently available that receive all of their operating power from a known 4-20 mA loop.
Some field devices include a transducer. A transducer is understood to mean either a device that generates an electrical output based on a physical input (such as a sensor) or that generates a physical output based on an electrical input signal (such as an actuator). Typically, a transducer transforms an input into an output having a different form. Types of transducers include various analytical equipment, pressure sensors, thermistors, thermocouples, strain gauges, flow transmitters, positioners, actuators, solenoids, indicator lights, and others.
Typically, each field device also includes communication circuitry that is used for communicating with a process control room, or other circuitry, over a process control loop. In some installations, the process control loop is also used to deliver a regulated current and/or voltage to the field device for powering the field device.
Traditionally, analog field devices have been connected to the control room by two-wire process control current loops, with each device being connected to the control room by a single two-wire control loop. Some analog field devices transmit a signal to the control room by modulating the current running through the current loop to a current that is proportional to a sensed process variable. Other analog field devices can perform an action under the control of the control room by controlling the magnitude of the current through the loop. In addition to, or in the alternative, the process control loop can carry digital signals used for communication with field devices. Digital communication allows a much larger degree of communication than analog communication. Moreover, digital field devices do not require separate wiring for each field device. Field devices that communicate digitally can respond to and communicate selectively with the control room and/or other field devices. Further, such devices can provide additional signaling such as diagnostics and/or alarms.
In some installations, wireless technologies have begun to be used to communicate with field devices. Wireless operation simplifies field device wiring and setup. One particular form of wireless communication in industrial locations is known as wireless mesh networking. This is a relatively new communication technology that has proven useful for low cost, battery-powered, wireless communication in commercial measurement applications. Wireless mesh networking is generally a short-range wireless communication system that employs low-power radio-frequency communications and is generally not targeted for long distance, plant-to-plant, pad-to-pad or station-to-station communications.
In general, wireless radio-frequency communication requires the use of an antenna. In such harsh industrial settings, the antenna is a relatively fragile physical component. Moreover, should the antenna break off, communication to the field device itself may be compromised. If the antenna seal to the housing is damaged or degraded (for example by UV exposure or hydrolytic degradation) the environmental seal can fail and cause damage to the field device. Generally, the antenna must extend above the transmitter housing (which is typically metal) and must withstand high vibration levels, impact, and extreme temperatures while maintaining a weather-tight seal and 90° adjustability.
Attempts have been to made provide adjustable antennas where the antenna is allowed to rotate freely when a set screw is loosened. However, it is believed that such unfettered rotation may allow cables within the antenna to twist and become damaged. Known wireless field devices generally employ an omni-directional dipole antenna. In order for two omni-directional dipole antennas to have optimal communication, they must be mounted with the same polarization (orientation). Further, vertical polarity is generally preferred for omni-directional antennas. Accordingly, adjustable antennas for field devices have been provided but it is believed that such adjustable antenna designs do not adequately protect the cabling within the antenna.
Providing an industrial field device having a rugged wireless communication antenna that better protects internal cabling would advance the art of wireless field device communications.